Washington: The US understands the concerns that Pakistan has, but it also has plenty of concerns with regard to Islamabad, a top American official said on Thursday.

"We have been in constant and intense dialogue with our Pakistani counterparts. We understand the concern. Frankly, there's plenty of concern on the American side as well," State Department spokesperson Victoria Nuland told reporters.

Nuland appeared to be upset that Pakistan has not joined the CENTCOM investigation into the November 26 incident in which 24 Pakistani soldiers were killed in a NATO air strike.

"When this tragic incident happened you know that every senior American called every senior Pakistani, from the President to the Secretary on down, to express our condolences and to pledge a full, transparent investigation," she said.

"So we are doing that investigation now. We invited the Pakistanis to participate with u in the investigation. And frankly, that would have been the best way for them to have eyes on and to ensure the credibility of what we were up to. Government of Pakistan declined to participate," she said.

"We obviously remain open to that kind of participation. But we also are very firmly committed to understanding what happened here, because we have had in the past similar difficulties coordinating and communicating on the border," she said.

"We have got to get to the bottom of what happened. We've got to learn lessons because, frankly, we need to continue to cooperate here. We can't -- either of us, Pakistan, United States, or Afghanistan, frankly -- allow these areas to remain safe havens for terrorists. So we've got to work this out and that's what we're committed to doing," Nuland said.

"But I'm not going to prejudge this report until it is out and until we've had a chance to talk to the Pakistanis about it, work with them on it, and learn lessons from it and improve procedures as a result of it," she said.